In a small radio device, for example, a mobile wireless terminal, etc., higher performance and miniaturization are required and for an antenna, miniaturization for enabling the antenna to be incorporated in a cabinet of a mobile wireless terminal, etc., and a wideband characteristic compatible with various applications are demanded.
As such an antenna, for example, a small wideband antenna described in FIG. 12 is known (for example, refer to Patent Document 1). That is, this small wideband antenna is formed by bending both ends of an element toward a feeding part in a state in which a full-wave loop antenna 101 is brought close to a ground plate 102; a current distribution is small on the ground plate 102 and the effect when the user approaches the antenna is hard to receive and in addition, directional characteristics responsive to an incoming wave can be realized. Numeral 103 in the figure denotes a wireless circuit.
As the antenna described above, for example, an antenna as shown in FIG. 13 is also known (for example, refer to Patent Document 2). That is, in this antenna, preceding portions from left and right about a one-eighth wavelength of a feeding point 202 of a folded dipole antenna 201 of half-wave parallel line with both ends short-circuited are bent vertically and horizontally as bilateral symmetry to form an antenna element 203 of a double folded structure. According to the antenna of the configuration, parameters for determining the antenna structure, particularly spaces s and h and a strip width ratio (w1/w2) are adjusted, whereby impedance matching over a wide band is made possible.
Further, as such an antenna, for example, an antenna as shown in FIG. 14 is also known (for example, refer to Patent Document 3). That is, this antenna is an antenna appropriate for being incorporated in a radio unit where multiple resonances and impedance adjustment are possible. Power is fed into a first antenna 301 of folded monopole type and a second antenna 302 of monopole type with the tip open, a short-circuit part is provided in an intermediate part of the first antenna 301, and the total length of a going path from a feeding point 303 to a fold point and a return path to a ground point 304 is a half-wavelength of resonance frequency. In the antenna, the second antenna 302 is made to branch between the feeding point 303 and the short-circuit part and the whole element length is made to correspond to a quarter-wavelength of resonance frequency; on the other hand, the first antenna element 301 also functions as a stub of the second antenna element 302. Numeral 305 in the figure denotes a board.    Patent Document 1: JP-A-2002-43826    Patent Document 2: JP-A-2004-228917    Patent Document 3: JP-A-2006-196994